Semi-interpenetrating Polymer Networks Composed of Poly(ethylene

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24 Semi-interpenetrating Polymer Networks Composed of Poly(ethylene terephthalate) and Castor Oil Synthesis, Structure, Physical Properties, and Crystallization L. W. Barrett , L. H. Sperling , J. W. Gilmer , and S. G. Mylonakis 1

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Center for Polymer Science and Engineering, Materials Research Center, Whitaker Laboratory 5, Lehigh University, Bethlehem, PA 18015 EniChem America, Inc., Research & Development Center, 2000 Cornwall Road, Monmouth Junction, NJ 08852 1

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Semi-interpenetrating networks (semi-IPNs) have been produced from castor oil and poly(ethylene terephthalate) (PET) by a synthetic method that involves bond interchange reactions that form partial miscible copolymers. Based on the energy absorbed during tensile fracture, the composition that contains 50 wt% PET has over 10 times greater toughness than the constituent PET. The crystallization kinetics of PET were enhanced by the presence of castor oil, both in cooling from the melt state and heating from the quenched glassy state. In conjunction with the chemical nucleating agent (sodium benzoate), PET compositions were even further enhanced in terms of their crystallization characteristics. Semi-IPNs with up to 30 wt% castor oil polyurethane displayed a higher fraction of PET crystallinity than neat PET, but had a lower melting temperature. Greater amounts of castor oil in these semi-IPNs significantly disrupted PET crystallinity and resulted in lower and broader melting and crystallization transitions and overall loss in crystallinity. 0065-2393/94/0239-0489$08.00/0 © 1994 American Chemical Society Klempner et al.; Interpenetrating Polymer Networks Advances in Chemistry; American Chemical Society: Washington, DC, 1994.

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INTERPENETRATING POLYMER NETWORKS


I N T E R P E N E T R A T I N G P O L Y M E R N E T W O R K R E S E A R C H THAT INVOLVES naturally

funetionalized triglyceride oils has been a topic of great interest at Lehigh University (1-9). These oils are prepared by nature with multiple chemical functionality, and are thus a renewable resource that provides an alternative to petroleum as a chemical feedstock. The oils may be cross-linked to form an elastomeric material, usually by step-growth reactions, which has prompted their use in interpenetrating polymer networks (IPNs) along with chain-growth polymerized plastics. Castor oil in particular has received much attention, primarily because of its status as a large-scale commercial product (J, 2). Other funetionalized triglyceride oils are numerous, but none is produced in the same quantities as castor oil. Some of these oils, however, are promising for the future because of their high functionality and the ability to cultivate the precursor oil-seed plants in arid climates at economical yields (10, 11). Barrett and Sperling (12a, 12b) studied semi-IPNs made from castor oil and poly (ethylene terephthalate) (PET). In the prior I P N literature, primar ily amorphous plastics were used. P E T is a departure from that norm in that it is a semicrystalline thermoplastic. Castor and other triglyceride oils with other polymers have mostly been limited to use as a plasticizer or stabilizer (11, 13-16). Because P E T is semicrystalline, the morphology of the semi-IPN becomes more complicated, which introduces crystallinity as a factor to be considered. Semicrystalline P E T has outstanding material properties that are required for an engineering plastic. However, the crystallization rate of P E T is slow, which forces long cycle times in injection molding applications. Crystalline P E T injection molding grades tend to have lower molecular weight, which increases the crystallization rate and improves melt flow, but the resulting material can be quite brittle and is nearly always sold as a composite with up to 40-vol% chopped glass fiber for increased toughness (17). To address these drawbacks of P E T , it was thought that by making semi-IPNs with funetionalized triglyceride oils, toughness as well as crystal lization rate could be improved. The present chapter reviews and extends the research of Barrett and Sperling (12a, 12b), with an emphasis on the entire composition range: basic characterization, mechanical behavior, and crystal lization behavior.

Description of Components Naturally Funetionalized Triglyceride O i l IPNs. The exten sive use of castor oil in I P N research began in 1977 with sulfur-cross-finked castor oil that was swelled with styrene and divinylbenzene (8) or methyl methacrylate and glycol dimethacrylate (9) and then polymerized. This form of cross-linked triglyceride, known as factice, makes use of the unsaturated bonds in the triglyceride oil, and can be made with any unsaturated oil (18). The uniqueness of castor oil does not he in its ability to form factice,

Klempner et al.; Interpenetrating Polymer Networks Advances in Chemistry; American Chemical Society: Washington, DC, 1994.

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24. PET-Castor Oil Semi-IPNs however, and subsequent to this early work, the hydroxyl functionality was exploited to form polyester or polyurethane networks. Castor oil polyester and polyurethane IPNs with polystyrene were studied at Lehigh University during the late 1970s and early 1980s (4-9), and today, research on castor oil polyurethane IPNs with acrylic as well as other polymers is active worldwide (19-22). Commercial castor oil consists of triglycerides that contain 90% ricinoleic acid residues and 10% nonfunctional acid residues, so that castor oil has an effective hydroxyl functionality of 2.7 (23-25). By reaction of the hydroxyls with a difunctional reactant, castor oil may be cross-linked to form a soft elastomer, with a glass transition temperature, T , as low as —50 °C (4, 5), dependent on the cross-linker used. Utilizing a dibasic acid, a polyester network can be formed; however, this reaction releases a condensation product: water. Sebacic acid ( H O O C [ C H ] C O O H ) has often been used as the dibasic acid because it is derived from castor oil (26). If instead a diisocyanate is used, a polyurethane network can be made without the release of any byproduct. Because of the difficulty involved in making a void-free material when water is released during cross-linking, hybrid polyesterurethane networks have been made, where the castor oil polyester is polymer ized to just below the gel point, all the water is driven off, and then the material is cross-finked with a diisocyanate. The formation of polyester or polyurethane networks has two distinct advantages over factice. Because the step growth reaction does not interfere with radical chain polymerizations, simultaneous polymerization with mono mers such as styrene and acrylics is possible. The other advantage is that the triglyceride oil double bonds remain intact in the polyester or polyurethane networks, which allows staining for electron microscopy imaging of the phase structure in the IPNs formed. These characteristics of triglyceride oils have made them invaluable in the study of morphology development in IPNs. Many other funetionalized triglyceride oils, such as vernonia, lesquerella, crambe, and linseed, as well as their epoxidized analogs, also have been used in the past to form IPNs with chain-polymerizable monomers, such as styrene and acrylics (2, 3, 5, 23, 27, 28a, 28b). Vernonia oil, a naturally epoxidized oil, was combined with P E T to make semi-IPNs in research (28b) parallel to that described in this paper. These funetionalized triglyceride oils, although not yet commercially available, offer different characteristics than castor oil, both in terms of the chemistry involved and the physical properties and morphol ogy development of the IPNs. IPNs may, in general, be formed by two methods (29). In the sequential method, a cross-linked network is formed, then swelled with the monomer and cross-linker of the second component and polymerized to create the interpenetrating network. If the two components polymerize by noninterfering routes, then the I P N may be formed by the simultaneous method, where monomers and cross-linkers of both materials may be mixed and polymerized g

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N ITERPENETRATN IG POLYMER NETWORKS

simultaneously. Prior triglyceride oil I P N research has proceeded by polymer izing both components from their monomelic state. In this way, the compo nents are initially miscible either as monomers (simultaneous method) or as a swollen network (sequential method) and then phase separate as polymeriza tion and cross-linking occur. It is the relationship between cross-linking and phase separation kinetics that controls the final I P N morphology (30a). In the present research, fully polymerized P E T is the starting material, which puts some constraints on the I P N formation process. P E T may be considered as the condensation product of terephthalic acid and ethylene glycol. Because the castor oil would polymerize with terephthalic acid in competition with ethylene glycol, simultaneous semi-IPN formation from monomers is not possible, because noninterfering polymerization routes are required. Although P E T and castor oil polyester network are both polyesters, P E T is aromatic and castor oil is aliphatic, and the two are immiscible. Thus, castor oil will not dissolve in P E T nor will polymerized P E T swell a castor oil network. These factors make sequential I P N formation impossible. To control the I P N morphology of P E T and castor oil network, the two components must first be either miscible or well mixed, and then be allowed to phase separate as the castor oil cross-linking occurs. The discovery that continued heating of castor oil and P E T results in a miscible mixture—a consequence of bond interchange reactions that form a compatibilizing copolymer (12a)—solved the problem of initial immiscibility. Because P E T contains ester groups and castor oil contains both ester and hydroxyl groups, either ester-ester or hydroxyl-ester interchange (or both) take place (12a, 30b). In either case, the resultant semi-IPN is a hybrid structure in which the cross-linked or uncross-linked materials may be copolymers rather than pure components. Thus, the extent to which the bond interchange reactions take place affects many aspects of the resulting semiI P N . This method, wherein miscibility between the two components is affected by bond interchange copolymer formation, is potentially applicable to many other systems where bond interchange takes place and should make interesting new IPNs and semi-IPNs. Crystallization of Poly(ethylene terephthalate). P E T is a widely utilized polymer because of its high strength, thermal stability, good barrier properties, low price, and good solvent resistance. P E T is established as a primary material in bottles, textiles, recording tape, and packaging film. Injection molding is, however, one area in which, until recently, P E T has not been extensively utilized due to a slow crystallization rate that results in poor dimensional stability of molded parts. The crystallization rate of P E T on cooling from the melt to the glass is considerably slower than the rate of poly(butylene terephthalate) (PBT) possibly due to difficulty in chain fold formation (Runt, J. P., private communication). The maximum crystallization rate of P E T occurs at about 180 °C, which is halfway between its glass-transi-



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24. PET-Castor OU Semi-IPNs tion temperature (approximately 70 °C) and its equilibrium melting point (about 285 °C). At 90 °C, a typical mold temperature used i n injection molding polyesters, the rate of crystallization for P E T is virtually zero. Although one method to improve the moldability of P E T is to raise the temperature of the mold, at temperatures greater than 110 °C it is no longer possible to use water to heat the mold, so oil must be utilized instead. Because most molders do not wish to switch to oil heating to mold P E T , nucleation agents and mobility enhancers have been developed that allow molding at temperatures of 100 °C or lower. Thus, much of the prior art that had been developed in the area of P E T molding concerns the development of nucleating agents that are effective to induce rapid P E T crystallization at high temperatures when cooling from the melt temperature to the mold tempera ture (31). Developments in the 1970s and early 1980s in P E T nucleation (32-35) involved the use of aliphatic and aromatic sodium carboxylates to nucleate reinforced P E T . Unlike the traditional inert solid physical nucleation agents such as talc, which work by an epitaxial mechanism, these chemical nucle ation agents work by dissolving in P E T . Extensive nucleation technology that has been developed for P E T involves the use of low molecular weight alkali metal carboxylates (36-42) as well as polymeric nucleants that contain pendant alkali metal carboxylates (43-45). Technology developed by Deyrup (33) describes the combined use of a nucleant, "a sodium or potassium salt of a hydrocarbon acid containing between about 7 and 25 carbon atoms, or with the sodium or potassium salt of an organic polymer which contains pendant carboxyl groups . . . and a mobility enhancer—a low molecular weight organic plasticizer (for example, ketones, esters, or sulfones). A filler is also added for the purpose of reinforcement of the resin. The presence of a chopped glass fiber in injection moldable PETs markedly increases both the tensile strength and modulus of the P E T and results in improved toughness. The use of carbonyl-containing plasticizers, such as esters and ketone, i n reinforced P E T in conjunction with selected sodium or potassium carboxyl ates enhances the rate of crystallization as the molded part has nearly cooled to the mold temperature (33, 44). Additional mobility enhancers that have been developed include mixtures of oligoethers and oligoether segments that are employed jointly to synergistically increase the mobility of the P E T (46, 47). The addition of small amounts of polyolefins such as polyethylene also enhances the mobility of P E T in the melt, as well as improves its toughness (48, 49). Research by Biebuyck et al. (50) details the nucleation of P E T by a variety of different basic salts of the alkali metals including carboxylic, phenolic, phosphonic, phosphinic, and sulfonic salts. This study describes the use of a range of ionizable metal salts that are able to activate the P E T chain end and thus are useful in the nucleation of P E T . The alkali metal salts of various metal oxides also nucleate P E T (51). The mechanism by which alkali



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metal salts function to nucleate P E T was best addressed by the spectroscopic study undertaken by Dekoninck et al. (52), who were able to demonstrate by Fourier transform infrared (FTIR) spectroscopy that a basic sodium salt reacts with the ester to cleave the P E T chain and to form a sodium carboxylate chain end. The presence of sodium on the end of the P E T chain then facilitates nucleation. If the P E T is annealed in the melt for extended periods of time, it gradually loses its nucleating ability. This loss occurs by the reaction of two neighboring sodium carboxylate chain ends to form disodium terephthalate—a very poor nucleating agent for P E T . Most group I carboxylates are effective as nucleants for P E T along with many other group I salts. Two examples of group I carboxylates that were found to be totally ineffective as P E T nucleants are disodium terephthalate and sodium parahydroxybenzoate. The precise reason that some salts are more effective than others is not understood, but is believed to depend, at least in part, on the ability of the salt to react with P E T and transfer the group I metal to the chain end. This capability could involve such factors as solubility of the salt in molten P E T , the heat of fusion of the salt crystal, and the relative basicity of the salt. Finally, chemical nucleation with ionic groups on chain ends is transfer able to polymers other than P E T . Legras et al. (53, 54) have shown how polycarbonate and poly (ether ether ketone) ( P E E K ) can be nucleated by the presence of ionic groups on the chain end. This mechanism presumably would be extendible as well to other high-performance condensation poly mers, such as polyimides, poly (phenylene ether), and high-temperature polyesters. S u m m a r y . Until now, triglyceride oil I P N research has concentrated on morphology and physical properties of amorphous compositions. The large amount of work that has been done in this field has expanded the knowledge base regarding how to control phase continuity and domain size and how to correlate morphology and physical properties. To a large extent, however, the use of semicrystalline polymers in IPNs has been ignored. Because P E T is a semi-crystalline polymer, the effect of crystallinity on I P N morphology and physical properties becomes a significant new variable to be considered. In this chapter, the effectiveness of castor oil to improve the crystallization characteristics of P E T is considered, as is the crystallinity of PET-castor oil network semi-IPNs. In the present research, only a small number of the questions related to crystallizable IPNs have been addressed, and perhaps more questions have been raised by the work than have been answered. It is hoped that interest in the area of semicrystalline IPNs may be stimulated by this work. Prior I P N systems were primarily based on miscible monomers that polymerize to form immiscible networks and were produced by either



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24. PET-Castor Oil Semi-IPNs simultaneous or sequential methods of I P N synthesis. In the present re search, castor oil semi-IPNs have been produced with fully polymerized P E T by taking advantage of bond interchange reactions to create an initially miscible mixture prior to cross-linking and subsequent phase separation. The resultant structure contains a significant fraction of copolymer, both in the network as well as in the uncross-linked portion. Although in this chapter just enough copolymer to create a miscible mixture was utilized, the extent of copolymerization becomes another synthetic variable that may be controlled to tailor structure, morphology, and physical properties. Creation of I P N materials from fully polymerized immiscible polymers through a bond inter change copolymerization process, followed by cross-linking, is a concept applicable to many other systems. This concept opens the possibility to create novel I P N compositions and microstructures.

Experimental Details Materials and Syntheses. Poly(ethylene terephthalate) was supplied by EniChem America, Inc. with intrinsic viscosity of 0.60 dL/g, as measured in 60:40 by weight tetrachloroethane:phenol. This viscosity corresponds to viscosity average molecular weight of ~ 13 kg/moi (55). The P E T was dried at 120 °C in a vacuum oven for at least 48 h prior to use. P E T may be polymerized further in the solid state at temperatures just below the melting point under vacuum (56-58). High molecular weight P E T of ~ 40 kg/mol (viscosity average) was produced in this manner by annealing fully dried P E T pellets for 72 h at 240 °C under vacuum. Castor oil was supplied by CasChem, Inc., and contains triglycerides in which some 90% of the acid residues are the hydroxyl-bearing ricinoleic acid. Toluene 2,4-diisocyanate (TDI) was purchased from Aldrich. Hydrogenated methylene diphenyl diisocyanate ( H M D I ) or 1,r-methylene-bis(4-isocyanatocyclohexane), commercially available as Desmodur-W, was supplied by Mobay Corp. Monosodium terephthalic acid (MSTA) was prepared by dissolving tereph thalic acid in dimethyl sulfoxide (DMSO) and stirring in the presence of insoluble sodium hydroxide pellets at room temperature. In this heterogeneous reaction, when a sodium hydroxide molecule is able to neutralize one of the earboxylie acids on terephthahc acid, the monosodium terephthalic acid precipitates from D M S O . Monosodium terephthalic acid is insoluble in both D M S O and water, and is therefore easily purified from the water-soluble sodium hydroxide and terephthalic acid, which is DMSO-soluble and water insoluble. Elemental analy sis, performed by Galbraith Laboratories, Inc. (Knoxville, TN) yielded 50.7-wt% carbon, 4.0-wt% hydrogen, 29.9-wt% oxygen, 3.6% sulfur, and, by difference, 11.8-wt% sodium, as compared to calculated values of 51.1-wt% carbon, 2.7-wt% hydrogen, 34.0-wt% oxygen, and 12.2-wt% sodium. The sulfur detected in the MSTA material was apparently due to incomplete separation from the D M S O solvent. Castor oil monosodium terephthalate (COMSTA) was prepared by direct esterification by heating castor oil with monosodium terephthalic acid at 180 °C


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for 10 h. The reaction scheme is

This reaction was not carried to completion, and the product had to be filtered from the remaining MSTA. Fourier transform infrared spectroscopy of the resulting COMSTA mixture showed new absorbance peaks at 1100 and 1240 c m , where the benzoate functionality absorbs strongly. Elemental analysis gave 73.1-wt% carbon, 9.8-wt% hydrogen, 15.2-wt% oxygen, and 1.0-wt% sodium; calculated values were 70.8-wt% carbon, 9.7-wt% hydrogen, 17.4-wt% oxygen, and 2.1-wt% sodium. Based on the sodium analysis, the mixture as used con tained about 50% COMSTA; the remainder was castor oil. Two methods of mixing and reacting P E T and castor oil were used. The first method used a Brabender batch melt mixer; a flask with magnetic stirring was used in the second method. The Brabender mixtures of P E T with the castor oil or prepolymers were made at 290 °C in a 50-mL torque rheometer batch mixer (Brabender Plasticorder) equipped with sigma blades. The P E T is first added and allowed to melt, after which the oil is added by syringe. The compositions made by this method all contain 90-wt% PET and 10-wt% castor oil product. The mixing was carried out for approximately 8 min with nitrogen purging. Samples of the material were removed from the mixer at specific times and quenched in ice water, but the bulk of the P E T compositions were allowed to cool slowly at room temperature, which resulted in highly crystalline materials. Semi-IPNs were also made in the Brabender mixer by first melting the PET, and then adding the premixed castor oil and T D I prepolymer. The isocyanate-hydroxyl reaction takes place almost instantly at the mixing temperature and forms the castor oil network in the presence of PET. Due to the viscosity mismatch between molten P E T and castor oil, blends that contain more than 10-wt% oil were made by a different method, which shall be described next. A wide range of compositions were made by the second method, which utilizes a flask and magnetic stirring. A time-temperature schematic that summa rizes this synthesis procedure is shown in Figure 1. First, the requisite amount of P E T was melted in castor oil in a flask at 280 °C under nitrogen atmosphere and with stirring; then the temperature was raised to 300 °C. With continued stirring at 300 °C, the initially immiscible mixture becomes miscible to the point of optical clarity. At this stage, the heat source is removed and the miscible mixture is allowed to cool to 240 °C (cooling below this temperature results in crystalliza tion and precipitation of the P E T from solution). The required amount of diisocyanate cross-linker ( H M D I ) is then rapidly added by syringe and the - 1


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Figure 1. Schematic diagram that depicts the synthetic procedure used in production of PET-castor oil-HMDI urethane network semi-IPNs. Axes are not drawn to scale.

solution is mixed quickly and poured into molds. The hydroxyl-isocyanate reac tion occurs rapidly at 240 °C, so the mixture must be poured into the mold within less than 1 min, before the castor oil gel point has been reached. After pouring, the molds are placed in an air oven at 100 °C for 18 h, where the cross-hnking reaction is brought to completion. Measurement Techniques. Differential scanning calorimetry (DSC) was done using a differential scanning calorimeter (Mettler TA-3000) in scanning (20 °C/min) and isothermal modes. The sample size was kept close to 15 mg. Quenched samples were first melted in the D S C for 3 min at 300 °C, then quickly taken out and submerged in liquid nitrogen. The samples were then reheated at 20 °C/min to measure the glass transition, crystallization, and melting temperatures. Mechanical testing was done using a material test system (Instron) interfaced to a microcomputer (IBM PS/2). The general test procedure and sample prepara tion is described in the ASTM D-1708 microtensile test method. The strain rate was 112%/min, which was calculated from 25-mm/min displacement for speci mens of gauge length 22.25 mm. Load versus displacement data was converted directly to stress versus strain without making any corrections, because it is assumed that strain outside the gauge section was minimal. The tensile strength, Young's modulus, and percent elongation were calculated in a straightforward manner per the ASTM standard, and the energy to break was calculated from the area under the stress-strain curve.


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Fourier transform infrared (FTIR) spectroscopy was carried out using a spectrometer (Mattson Polaris) interfaced to a microcomputer (Zenith-386). Sample preparation was generally performed by making a l-wt% solution of the material to be analyzed in trifiuoroacetie acid (TFA), followed by dropwise addition of the solution onto silver chloride (AgCl) plates and evaporation of the TFA. In general, 64 scans from 800-4000 cm were taken at 2-cm ~ resolution and averaged. Because the thickness of the film on the A g C l plates was unknown, quantitative methods required determination of the ratio of peak heights to a standard of known composition. Downloaded by UNIV OF TENNESSEE KNOXVILLE on May 19, 2016 | http://pubs.acs.org Publication Date: May 5, 1994 | doi: 10.1021/ba-1994-0239.ch024

1

Results The flask mixture method was used to prepare the entire range of castor o i l - P E T compositions in increments of 10 w t % by weight. In these materials, the P E T and castor oil were mixed and heated to the point of miscibility; then the castor oil was cross-linked with H M D I to form a polyurethane network. The resultant materials were yellow-white in appearance and ranged in properties from elastomeric to plastic, dependent on composition. Mechanical Properties. Selected stress-strain curves of these ma terials are shown in Figure 2, and calculated values given in Table I. The initial P E T material has a low molecular weight ( M = 12.9 kg/mol), at or just slightly below the critical entanglement molecular weight (59), so that v

Strain (m/m) Figure 2. Stress-strain curves for a range of PET-castor oil-HMDI polyurethane network semi-IPN compositions. (Reproduced with permission from reference 12b. Copyright 1993)


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Oil Semi-IPNs

Table I. Calculated Values from Tensile Stress-Strain Data PET


(%) 0 10 20 30 40 50 60 100 100 (HMW) 50 (HMW)

Tensile (MPa)

Modulus (MPa)

Elongation (%)

Energy

1.2 1.6 4.3 8.4 10.6 13.5 10.9 16.4 55.0 18.2

4.1 5.7 14.7 27.7 53.3 68.5 192.8 1269.0 1700.0 116.7

116.3 139.1 132.7 116.9 85.3 93.6 31.0 1.1 39.0 119.8

850 1,216 3,410 6,121 6,589 10,867 3,251 114 12,500 16,000

(j/n?)

NOTE: Standard deviation in measured values was 15% (of the mean) for tensile strength, 25% for modulus, 17% for elongation, and 22% for energy density. SOURCE: Data are taken from reference 12a.

this P E T became brittle when crystallized. Added to this problem is the bond interchange with castor oil that would further reduce the average molecular weight. As a result, compositions with over 60-wt% P E T were too brittle to be tested. Despite these effects, the intermediate composition semi-IPNs have much better toughness than the materials from which they were made. The 50-wt% P E T semi-IPN clearly stands out with elongation close to that of the castor oil elastomer and tensile strength almost as high as the neat P E T . When a similar 50-wt% semi-IPN was made with higher molecular weight P E T ( M = 40 kg/mol), the mechanical properties were improved, but not nearly as much as the improvement in the neat P E T by the molecular weight increase (Figure 3). Here the reduction in molecular weight of P E T caused by bond interchange reactions with castor oil must be considered. Starting with P E T of M = 12.9 kg/mol, after reaction with castor oil, the viscosity average molecular weight dropped to 1450 g/mol. This drop repre sents nearly a ninefold decrease in molecular weight. Although this drop may be overestimated due to the dilute solution viscosity method of molecular weight measurement (the PET-castor copolymer may have tighter coils and lower solution viscosity than neat P E T at equal molecular weight), the evidence suggests that P E T chains are severed. Thus, although at 40 kg/mol, the neat P E T is well above the critical entanglement molecular weight and quite strong and tough, the eastor-PET semi-IPN may contain P E T with molecular weight below the critical entanglement molecular weight, and is thus only slightly improved over a semi-IPN made with low molecular weight PET. v

v

Values calculated from the stress-strain curves are tabulated in Table I, in which only the materials denoted by H M W were made with the high molecular weight P E T . Although the modulus surely increases with P E T



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Strain (mlm) Figure 3. Stress-strain curves that compare 50:50 PET.castor oil-HMDI polyurethane network semi-IPNs made with high (M = 40 kg/mol) and low (M = 13 kg/mol) molecular weight PET. v

v

content, it takes a large jump between 40- and 60-wt% P E T , which is indicative of a change in the continuous phase from castor o i l - H M D I polyurethane (low modulus) to P E T (high modulus). The energy to break for the samples is greater for the semi-IPNs than for either of the constituent materials, even in the case where the tough high molecular weight P E T was used. The energy required to break the 50-wt% low molecular weight P E T composition is over 10 times that for the neat P E T used, and is comparable to that for castor oil-polystyrene IPNs (60). G l a s s - T r a n s i t i o n B e h a v i o r . Although the modulus versus composi tion data indicate which component predominantly constitutes the continuous phase, a more detailed picture of phase structure may be obtained by studying the glass transition. In Figure 4, the glass transitions as measured by D S C for the components and their mixtures are shown, both in conventional heat flow versus temperature and derivative of heat flow formats. In the 50:50 castor oil: P E T mixture, which was sampled just before the diisocyanate cross-linker was added but after the solution became clear at 300 °C, two phases were present: one with a glass-transition temperature that matches that of castor oil and the other with a transition temperature near that of



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Figure 4. DSC glass-transition curves for the 50:50 PET.castor oil-HMDI polyurethane network composition and its components. Conventional DSC scans, with heatflowversus temperature (A), as well as derivative heat flows (B) are shown. (Reproduced with permission from reference 12h. Copyright 1993.)



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P E T , but shifted downward from 75 to 45 °C. When the diisocyanate was added to this mixture, which created cross-links between the hydroxyls present, the resultant glass transition was broadened to a range roughly from the start of the castor oil polyurethane transition to the end of the PET-rich phase transition observed in the mixture before cross-hnking. This behavior is common to microheterogeneous phase morphology, where a single broad glass transition is observed over the range that covers the transitions of the constituent materials (29). It should be emphasized that the glass transition reveals information about only the noncrystalline portion of the mixture. In these semi-IPNs, P E T is able to crystallize, so that a single glass transition could be observed when the P E T is highly crystalline. However, as the P E T content is reduced in the semi-IPN, so is its ability to crystallize to large extents. Thus, when a single glass transition is encountered in these semi-IPNs, it is most likely due to microheterogeneous morphology, not high crystalhnity of the P E T fraction. Derivative heat flow D S C plots that show a range of PET-castor o i l - H M D I polyurethane compositions are depicted in Figure 5. These curves reveal the transition from two-phase morphology (30-wt% P E T ) to micro heterogeneous morphology (50-wt% P E T ) and back again to two phases (70-wt% P E T ) as a function of composition. This behavior is the result of the effects of bond interchange on miscibility and the rates of cross-linking and phase separation, which determine the morphology.


i-IPNs Semi-


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503

Percent PET

Figure 6. Peak glass-transition temperature versus weight percentage PET for the range of PET-castor oil-HMDI polyurethane network semi-IPN compositions. The straight line represents the glass-transition temperature calculated through use of the Fox equation. (Reproduced with permission from reference 12a. Copyright 1991.)

Further analysis of the glass-transition behavior of the series of semi-IPNs reveals some interesting characteristics. Determining of the peak glass-transi tion temperature (the temperature corresponding to the minimum in deriva tive heat flow) of a wider range of compositions and plotting of these temperatures versus weight percent P E T revealed the values shown in Figure 6. The straight line depicted in Figure 6 shows the value of glass-transition temperature calculated by the Fox equation (62), which correctly predicts the data for the high-PET end of the composition. Because the Fox equation was derived for prediction of the glass-transition temperature of miscible blends or random copolymers, this agreement suggests that the PET-containing phase is a miscible mixture, or copolymer, formed between P E T and castor oil. At the low-PET end, the peak glass-transition temperature is virtually constant and equal to that for castor o i l - H M D I polyurethane, which suggests that the pure polyurethane is the predominant phase in that case.

Crystallization Behavior. Castor oil is compared with common nucleating agents in P E T compositions in Figure 7, where the normalized extent of crystallization is plotted versus temperature. In crystallization from the melt, 1 w t % of the chemical nucleating agent sodium benzoate effectively


504



Crystallization upon 20C/min Cooling

Temperature (C)

Figure 7. Extent of crystallinity versus temperature calculated from DSC data during cooling from the melt state (A) and heating from the glassy state (B) for PET and PET compositions that contain either l-wt% sodium benzoate, l-wt% calcium acetate, or 10-wt% castor oil.



BARRETT ET AL.

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24. PET-Castor OU Semi-IPNs improves the onset and peak crystallization temperature of P E T ; however, the effectiveness of castor oil is close behind and is, in fact, better than 1 w t % of the physical nucleation agent, calcium acetate. In crystallization from the glass, castor oil is significantly better than sodium benzoate, which in this case is no more effective than calcium acetate. Combinations of sodium benzoate and castor oil were investigated next. The castor oil and sodium benzoate may simply be mixed together with P E T , or else a new molecule can be created in which the sodium benzoate functionality is bonded to a castor oil molecule. The reaction scheme used to form such a hybrid molecule, castor oil monosodium terephthalate ( C O M S T A ) was shown earlier in eq 1. In Figure 8, D S C crystallization scans of P E T compositions that contain the bonded castor-sodium benzoate are compared with similar compositions wherein the castor oil and sodium benzoate are simply mixed. In crystallization from the melt, the simple mixture has a higher onset and peak crystallization temperature, whereas the C O M S T A composition has a much sharper crystallization peak. Table II gives some of the numerical crystallization data for these compositions, both in cooling from the melt and as heating from the quenched glassy state. The C O M S T A composition has a greater heat of fusion ( Δ Η ) than the simple mixture in cooling and neat P E T in both heating from the glass and cooling from the melt. The theoretical heat of fusion for 100% crystalline P E T is 130 J/g (59), so the materials in Table II range in crystallinity from ~25 w t % up to 45 wt%. In crystallization from the glass, the simply mixed castor oil-sodium benzoate composition could not be adequately quenched to yield comparable data, due to rapid crystallization during cooling, and therefore is not included. Although the C O M S T A composition could be quenched in liquid nitrogen, it had a crystallization temperature 8 °C lower than the castor oil composition, which has a crystallization temperature ~ 20 °C lower than PET. The enthalpy of fusion measured for the synthesized series of semi-IPNs is plotted versus the amount of P E T in Figure 9. These materials have relatively high crystallinity as a result of the semi-IPN synthesis procedure, wherein they were held at 100 °C for 18 h, which effectively annealed the P E T . The semi-IPNs with 80-wt% and 90-wt% P E T actually have greater crystallinity per gram of P E T than similarly annealed neat P E T ; however, the crystalhnity of semi-IPNs with less than 60-wt% P E T falls off rapidly. Figure 10 shows the peak melting temperature upon heating and the onset and peak crystallization temperature upon cooling as a function of weight percent P E T for the semi-IPN series. The addition of 60-wt% castor o i l - H M D I polyure thane network depresses the P E T melting point by over 50 °C. The differ ence between crystallization onset and peak temperatures also becomes greater because there is less P E T in the semi-IPN; thus, the transition is broadened by the addition of the castor oil polyurethane.


506



Temperature (C) Β Figure 8. DSC heating scans that show crystallization from the melt (A) and glass transition and crystallization from the glassy state (B) for PET and compositions that contain castor oil or sodium benzoate. (Reproduced with permission from reference 12b. Copyright 1993 Society of Plastics Engineers.)


24.

BARRETT ET AL. PET-Castor Oil Semi-IPNs

507

Table II. PET Crystallization Data for Castor Oil or Sodium Benzoate Compositions Τ

Composition

Τ

An

g


cm

PET 10% Castor oil 1% Sodium benzoate 10% Castor + 1% sodium benzoate 10% COMSTA

45.9 58.1 52.4 49.4

202.2 215.0 222.5 221.8

36.5 40.6 33.3

55.9

219.5

38.7

133.6 109.7 117.2 a

a

102.1

N O T E : Δ H is the heat of crystallization as measured in cooling from the melt state; T is the peak crystallization temperature as measured in cooling from the melt state; Δ H is the heat of crystallization as measured in heating from the glassy state; T is the peak crystallization temperature as measured in heating from the glassy state. This sample could not be quenched effectively enough for comparable data. S O U R C E : Data are taken from reference 12a. m

cm

g

cg

a

Semi-interpenetrating Polymer Networks Composed of Poly(ethylene

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